Turbine rotors operating at high temperatures must be made of creep-resistant steel.
Steel containing 9% to 14% chrominum have good creep resistance but tend to seize. This tendency can give rise to operating defects in any of the parts of the rotor which are in direct contact with fixed parts, i.e. the inner or outer bearing surfaces of the bearings, and the bearing surfaces of the end stops, if any.
In order to prevent seizure phenomena and to improve the coefficient of friction, it is known to cover the rubbing parts with plates and shells of steel having a good coefficient of friction, as indicated in published European patent specification No. EU-A-68492.
However, the properties of such rotors may change in service. If the plates or the shells are fixed on by means of bolted assembles within the rotor itself, risks stem from stress concentrations at the bolts and, in particular, at their threads. If the plates or the shells are welded on, risks stem from degradation in the mechanical qualities of the rotor steel where it has been heated by the welding, which degradation is practically irreversible.
In accordance with the invention, a metal coating (preferably of chromium) is electrolytically deposited on the rubbing parts of the rotor.
The apparatus for depositing the metal enables very low coefficients of friction to be attained because of the uniformity of the deposited coating.
European published patent specification No. EU-A-37033 describes apparatus for depositing a metal coating on the hub of a rotor disk, said apparatus comprising a vat having a vertical axis 00' constituted by a cylindrical portion terminated by a conical portion, said vat being filled with electrolyte, first means disposed near the top of said vat for feeding the vat with electrolyte, second means situated at the bottom of the vat for emptying the electrolyte, third means situated between the second means and the first means for regenerating and circulating the electrolyte, fourth means for supporting a curtain of anodes made of the metal to be deposited and for rotating the electrolyte about the axis 00', and fifth means for applying current of given polarity to the curtain of anodes.